Pulse width modulation (PWM) control used in a power supply circuit raises or lowers input voltage by a change in duty ratio of a PWM signal.
FIG. 5 illustrates a structure example of a PWM control circuit. The PWM control circuit includes an error amplifier 50, a reference voltage generation circuit 60, a PWM limiter circuit 70, an oscillator 80 for generating a triangle wave, and a PWM comparator 90.
The error amplifier 50 amplifies a difference between feedback voltage Vfb and reference voltage and outputs voltage Verr.
The reference voltage generation circuit 60 generates the reference voltage and reference voltage Vref.
The PWM limiter circuit 70 controls its output voltage Vers by comparing the voltage Verr output from the error amplifier 50 and the reference voltage Vref with each other.
The oscillator 80 generates a triangle wave Vosc that is a signal needed for generation of a PWM signal.
The PWM comparator 90 outputs a PWM signal from the voltage Vers output from the PWM limiter circuit 70 and the triangle wave Vosc, generated in the oscillator 80.
In PWM control, when a duty ratio represented by a ratio of the pulse width of a PWM signal to the cycle of the PWM signal is higher than or equal to a certain ratio (80%), harmonic noise might be generated. Further, an element might be damaged by supply of excessive current.
Therefore, in order to perform PWM control without the above problems, it is necessary to perform limiter control for preventing the duty ratio of a PWM signal from being higher than a certain ratio.
Reference 1 discloses a method for controlling the duty ratio of a PWM signal by input of the highest duty ratio voltage to a comparator when voltage output from an error amplifier is higher than the highest duty ratio voltage as a method of limiter control.